add caching and clang

Author: ladyada

.travis.yml

@@ -1,19 +1,31 @@
 language: c
 sudo: false
-
-# Blacklist
-branches:
-  except:
-    - gh-pages
+cache:
+  directories:
+    - ~/arduino_ide
+    - ~/.arduino15/packages/
+git:
+  depth: false
+  quiet: true
 
 env:
   global:
      - PRETTYNAME="Adafruit PCA9685 PWM Servo Driver Arduino Library"
 
+addons:
+  apt:
+    sources:
+      - llvm-toolchain-trusty-5.0
+      - key_url: 'http://apt.llvm.org/llvm-snapshot.gpg.key'
+    packages:
+      - clang-format-5.0
+
 before_install:
    - source <(curl -SLs https://raw.githubusercontent.com/adafruit/travis-ci-arduino/master/install.sh)
+   - curl -SLs https://raw.githubusercontent.com/adafruit/travis-ci-arduino/master/run-clang-format.py > run-clang-format.py
 
 script:
+   - python run-clang-format.py -r .
    - build_main_platforms
 
 # Generate and deploy documentation